This invention relates to a control arrangement for earth-working machines and, more particularly, to time-proportioning control arrangements for controlling the machine in grade, slope, and/or the like axes.
Earth-working machines, such as graders, pavers and curb and gutter machines, typically have either servo valves or solenoid valves for controlling the operation of the hydraulic motors or cylinders performing various functions on the machines. For example, these valves may control the hydraulic rams for positioning the cutting blade, screed or slip-form of these machines in grade and/or slope axes, or these valves may control the steering of the machine.
Solenoid and servo valves are used in distinctly different types of control systems. Servo valves, a generally more expensive device, is a proportioning valve and is used in proportional control systems. Proportional control systems have the advantage that the hydraulic ram, which is driving a grader blade, for example, is driven at a high speed for large error signals and at a decreasing speed for decreasing error signals. The higher speeds at large error signals allow for immediate correction of the controlled element whereas the slower speeds for smaller errors signals prevent overshoot and cycling around the control point.
As a substitute for servo valves, solenoid valves have found use because they are generally less expensive than servo valves. Solenoid valves are more compatible to "open center" hydraulic systems than are proportional valves. Open center hydraulic systems are frequently used in mobile equipment to reduce oil heating and use simple, economical hydraulic pumps. Solenoid valves are essentially on-off devices used in on-off or bang-bang control systems. On-off systems are incapable of providing both high positioning speeds and narrow dead bands, i.e., the distance that the sensor must travel between turn on and turn off of the system. Unlike proportional control systems, on-off control systems move the hydraulic rams at a fixed speed. If on-off systems are operated at high speeds and narrow dead bands, the hydraulic ram is moved very fast in response to error signals and tends to be driven through the dead band of the system so that the hydraulic ram must be driven back again in the opposite direction. This operation results in overshoot and cyclical operation around the control point. To prevent overshoot and cyclical operation, it is necessary to operate on-off systems either at low speeds with narrow dead bands which, because of the slow speeds, result in inadequate compensation for error signals or at high positioning speeds with wide dead bands which permit too much error.